The present invention relates to the detection of gases using infra-red radiation.
In a known apparatus of this type, an emitter of infra-red radiation is situated at the focus of a mirror, which collimates the radiation and produces two beams that pass respectively through an object and a reference cell. The reference cell is sealed and contains usually dry air or nitrogen. The object cell is provided with two ports through which sample gas is pumped. The radiation emerging from the two cells is incident on a second mirror and is focussed on to an infra-red detector. A rotating chopper alternately allows energy from the reference and the object cell to reach the detector. A synchronizing device detects the position of the chopper and signals this position to controlling electronics, which process the pulses of information coming from the detector. The output of the system is determined according to the decrement in the energy of the beam which has passed through the object cell when compared to that emerging from the reference cell. If the object cell contains gas which absorbs infra-red radiation in the response band of the instrument, the radiation reaching the detector via the object cell will be reduced. The two alternate pulses of energy reaching the detector will therefore be of different magnitudes, and the processing electronics will output a D.C. voltage or current level which will be a measure of the decrement of energy transmitted through the object cell and hence a measure of the infra-red absorbing gas in the object cell. This technique is used extensively for the detection of many gases, notably hydrocarbons, which absorb radiation with wavelengths of approximately 3.4.mu..
Disadvantages of this known type of apparatus are the need for focussing optical parts that can be upset by any movement of the apparatus and the relatively low proportion of the energy emitted by the infra-red emitter that is usefully employed. The energy dissipated heats the apparatus, so cooling is often necessary. Moreover, the time taken for the apparatus to reach a working condition from switch-on is relatively lengthy.
It is an object of the present invention to provide an apparatus in which these disadvantages are reduced.
According to the present invention there is provided apparatus for detecting gases by absorption of infra-red radiation comprising a source of infra-red radiation arranged at one end of an optical path of the apparatus at the other end of which is an infra-red detector, a plurality of cells arranged to be moved, in turn, into the optical path, the cells being such that when a cell is positioned on the optical path the infra-red radiation passes through that cell before reaching the detector, and electronic means for comparing the infra-red radiation received by the detector when one cell is positioned on the optical path with the infra-red radiation received by the detector when a selected other cell is positioned on the optical path and for producing a signal dependent on the said comparison.
Preferably, the cells are mounted for rotary motion about an axis parallel to the optical path of the apparatus and are moved successively into the optical path by said motion. Alternatively, the cells could be mounted as a pendulum or slide and moved in that way.
In the preferred form, the cells are housed within an enclosure and include one or more sealed reference cells containing e.g. nitrogen, dry air or a known concentration of a gas the detection of which is sought and an object cell that has openings in its side wall near its two ends whereby an atmosphere to be tested is pumped or allowed to pass into the enclosure and is pumped through the object cell by the motion of that cell through the enclosure.
In at least its preferred form, the present invention has the following advantages over the known apparatus.
All focussing optics are dispensed with, thus giving an improvement in stability and robustness.
A much greater portion of the energy emitted by the infra-red source is utilised, thus allowing a reduction in the power supplied to the source for an equivalent signal at the detector. A reduction to approximately one fifth of that supplied to a conventional source is easily possible, thus easing considerably the heat dissipation and warm-up problems associated with conventional equipment. A useful output can be obtained from the device in under two minutes from initial switch-on which compares very favorably with the warm-up time of many conventional instruments.
The self-pumping action of the object cell allows the device to be used as a direct monitor of the atmosphere of any particular area, since gas may be allowed to diffuse from such an area to the enclosure. Thus the external pump required in the conventional device is not required. Alternatively, the sample may be obtained from a distant area by using an external pump to draw the gas to be tested into the enclosure. Thus the installation of the device may be simplified and easily tailored to the particular application.
As mentioned above, there may be more than two cells. The additional cells could well be filled with samples of a variety of gases of interest, for example, methane, propane, butane, ethane, each at the alarm level(s) of interest. An external switch would allow the user to select whichever cell(s) are appropriate for the particular application. A synchronizing device forming part of the said electronic means then signals the processing electronics whenever the selected alarm level cell is at the optic axis, and thus gain and zero levels may be adjusted automatically by the electronics allowing the device to operate simply as a comparator, thus giving an extremely reliable alarm function.
If required, the single, static, infra-red filter may be replaced by a dual or multiple pass-band filter, thus allowing zero span and alarm adjustments to be made by the processing electronics from the detector outputs derived from a single cell. This would prevent cell window obscuration from causing errors due to outputs taken from different cells. The multiple infra-red filter system could be driven in synchronism by any suitable mechanical or electrical drive system.